Your feeling of a lack of energy may have multiple reasons.
First of all most sedentary activities hardly provide the intense sensory stimulation of tennis.
Most have very busy lives and don't get enough sleep.
Most have fatigue from the mental strain of work/school, eye strain from too much computer time, and trying to balance all of their committments.
Tennis should be a welcome relief from the stress of everyday life.
But the combination of all that stress can leave you exhausted, and wondering how in the world you can take on the additional challange as somehing so physically and mentally demanding as tennis.
Ah, but the delight of getting out there and bashing that fuzzy yellow ball makes all right with the world.
So how to bridge that gap between arriving and optimal play?
Even if you "can't get there early to play", be sure to get into some form of dynamic warmup. Running, sideskipping, swinging your racquet all help to warm up your muscles. 5 minutes hitting against a wall before your court opens up can get the blood pumping and your hand/eye coordination focus improved.
I know there is a limit to the warmup and how agressive the shots you take, but you owe it not only to yourself, but out of respect, to your opponent, to play a good match. So extend the warmup another couple of minutes if you need to, to hit at least a few hard crosscourt shots, forays to the net on short balls, and even a hard down the line shot or two. Practice volleys, overheads and serves in your warm up just like the pros do. By this time you should be pretty warmed up and focused. You will also get added comfort by developing a pre-match ritual.
You mention that being nervous may be part of your problem. Don't worry about the match. Only concern yourself with one point at a time, then the match will take care of itself.
Before each point only focus on what type of serve or return you plan/expect and as far ahead of what to do with the next ball that likely will come your way as a result of the serve/return (eg. after your wide serve, expect to hit the reply to the open court). During the point intently focus on the ball.
Once you are into the rythmn of the match with intense focus you are where you want to be. Even down 4-1 or 5-2 remember that you are just down one break, and be confident you will break back and take the set if you continue to just concentrate on one point at a time.
I hope this helps.
Good luck!